Urgent security updates (known as Rapid Security Responses) are provided to allow faster response to certain security issues, including issues that could be exploited or reported, Apple explains in its Help Center.
In this case, Apple said in a note (new window) that the bug could allow arbitrary code to run when processing web content, meaning a malicious website could potentially install malware on your device.
Apple also says it is aware of a report that this issue may have been actively exploited.
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Apple device owners can download macOS Ventura 13.4.1(a), iOS 16.5.1(a), and iPadOS 16.5.1(a) updates for their Mac computer, iPhone, and iPad.
The Safari browser also has a security update to 16.5.2 on macOS Big and macOS Monterey operating systems.
To check if your devices are up to date, go to the Software update tab in the general settings.